1. Field of the Invention
The present invention relates to devices able to read magneto-optically and simultaneously all the parallel tracks recorded on a magnetic tape. It is notably applicable to magnetic tape systems for recording high-density digital data, known under the name SDCR (Static Digital Cassette Recording).
2. Description of the Related Art
The SDCR high-density magnetic tape reading/recording system was designed in the central R&D laboratory of the THOMSON-CSF company with the goal of producing a high-density magnetic tape recording system operating simultaneously on a set of parallel tracks, without using classic rotating heads. For this purpose the SDCR uses a highly original recording device incorporating a magnetic head having a set of integrated poles laid out matrix-fashion, and a Kerr effect magneto-optical read head of substantially monolithic structure. The write and read heads therefore enable parallel recording and reading of extremely high data density, despite the fact that the fabrication of the SDCR is very simple compared with rival systems.
More particularly, the Kerr effect read head includes, in a manner now known to professionals of the art, a prism comprising a stack of suitable materials. It is illuminated by a laser producing a flat polarized incident beam whose thickness is adapted to the dimension of the bits written on the magnetic tracks, and whose width is sufficient to cover the whole breadth of the magnetic tape. After reflection on the sensitive layer of the head, the polarization of the optical beam changes according to the direction of the magnetization induced by each recorded track of the magnetic tape. This polarization change is transformed into an intensity change, by a polarizer for example. The beam is then received on an array of sensors, of CCD (charge-coupled device) type for example. Each of the cells of this CCD then delivers a signal representative of the information recorded on each of the tape tracks.
The system described succinctly above has been divulged in more detail in a set of patents describing the global architecture of the system and certain special aspects of its fabrication. We can notably mention the following French patents filed by the applicant:    8917313 (28 Dec. 1989), publication no. 2 656 723,    84 07761 (18 May 1984), publication no. 2 564 674,    93 01407 (7 Feb. 1993), publication no. 2 701 332,    90 00546 (18 Jan. 1990), publication no. 2 657 100,    92 11146 (18 Sep. 1992), publication no. 2 696 037,    8614974 (28 Oct. 1986), publication no. 2 605 783,    87 14818 (4 May 1987), publication no. 2 622 335,    88 05592 (27 Apr. 1988), publication no. 2 630 853,    96 08393 (5 Jul. 1996), publication no. 2 750 787.
This system has enabled satisfactory prototypes to be built. Experiments on these prototypes have however revealed a number of problems whose resolution would enhance both the performance and working life of devices made according to this system.
One of these problems concerns the wear of the magnetic read head.
Although this read head is made from particularly abrasion-resistant materials, it is still subject to wear caused by the movement of the magnetic tape over it, which leads to progressive degradation of the head's performance and ultimately to its failure. This is moreover common to all systems in which there is contact between the read head and the magnetic tape, in particular the systems with rotating heads used today.